This invention relates to pneumatic tires for vehicles and more particularly to belted radial tires of the type having a cap ply overlaying the tire belt.
The expression "radial" when applied to tires means various tire constructions having a carcass with one or more reinforcement plies of tire cords extending from bead to bead in a substantially radial orientation i.e. the cords are oriented normal or substantially normal to the beads and to the crown centerline of the tire. In a mono-ply radial tire the carcass or body cords normally have a ninety degree bias angle, i.e., in the unshaped carcass they extend perpendicular to the planes of the beads. In a two-ply radial tire, the cords in each body ply are usually oriented at oppositely disposed small angles of up to 10.degree. with respect to the perpendicular to the bead planes. In four ply or heavier radial ply tire constructions, similarly opposed orientation of the cords in successive body plies is usually employed. All of these carcass constructions are contemplated to be within the scope and meaning of the expression "radial" as used herein.
Radial tires are generally constructed with a breaker or belt interposed in the crown region of the carcass and the thread for reinforcing the latter, such belt being composed of one or more layers or plies of tire cords which are generally inextensible and are made from such material as metallic wires, glass fibers, rayon, etc. In a mono-ply belt, the cords have a low bias angle of approximately 0.degree. , i.e., they are oriented substantially parallel to the planes of the beads and to the median equatorial plane or crown center line of the tire. If the belt is of a multi-ply construction, similar but opposed low bias orientations of the cords or cables generally at angles ranging up to approximately 30.degree. with respect to the median equitorial planes of the tire are employed in successive plies.
It is known in the art to provide an additional cover or cap ply of textile cords overlying the belt. For example, in U.S. Pat. No. 3,503,432 issued on Mar. 31, 1970 to L. Maiocchi, a cap ply is disclosed which is made of a heat shrinkable nylon cords. The shrinkage stresses developed in such cords are utilized to apply compressive pressure to the underlying belt plies and to absorb some of the stress present in the tire during operation. Similarly U.S. Pat. No. 3,850,219, issued on Nov. 26, 1974 to R. H. Snyder and U.S. Pat. No. 3,786,851 issued on Jan. 23, 1974 to H. Mirtain each disclose the use of such a cap ply.
The use of a cap ply endows a tire with additional travel comfort and safety, superior rolling resistance and in general improves the high speed characteristics of tires. The cap ply protects the metallic cords of the belt against the effects of humidity and reduces ply separation in the belt edges at elevated speeds.
The prior art cap plies are formed from a sheet of rubber like material having polyamide or polyester cords of double or triple filament yarn construction positioned therein. The properties of the cords determine the properties of their respective cap plies. The use of such cords in cap plies has been relatively successful because of the cords tear resistance, elongation and shrinkage properties. These cords also, however, determine the flexibility, heat dissipation characteristics, thickness and weight of the cap plies.
Certain problems arise with regard to the use of cap plies formed from such cords due to the resulting cap plies lack of flexibility, poor heat dissipation characteristics, bulk and heavy weight. The service life and operating characteristics of tires can be seriously effected by these factors.